Interference eliminating circuits



Jan. 20, 1953 l. H PAGE 2,626,349

INTERFERENCE ELIMINATING CIRCITS Filed Aug. 15, 1942 2 SHEETS-SHEET 1 //a E I. IE; j.

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INTERFERENCE ELIMINATING CIRCUITS Filed Aug. 15, 1942 2 SHEETS- SHEET 2 (usm rA/v'r oarPz/r voz. TA ai) Irving H Page Patented Jan. 2,0, 1953 UNITED yS'IA'TES PATENT OFF ICE (Granted 1under Title 1S."C0de (1952,),

6 "(laims.

VSBC.

'This invention `relates 'broadly 1to `a means for reducing or eliminating strong interfering 'signals `in radio reception `and more *particularly 'to such a means applicable to stadio echo ireceivers.

The modern system 'fof Vradio echo .range :inding vemploys a :transmitter :characterized in the ability .to 'produce :a series of vvsequential, rectangular-'shaped pulses, teach being fof very fshort duration :and separated fone :from the other .by fa comparatively long interval of time. .These'pulses are usually Asent out 'from the transmitter .in a unidirectional path rand upon "striking a remote obj ect lare.reflected-back to the Apoin'tof transmission or some other :point .as .'directedby'the angle of incidence. The echo received Lat veither one of these points will be `ofsuch Vlow intensity that an exceedingly high `gaiin receiver must zbe Vused to amplify the echo toa valuecapable'of producing a Idistinct signal'on thecathode ray :tube which is generally 'used in Va -receiverfof this class. In addition to 'the Yhghpgain clf1aracteristic `.the ;pre'sent receiver must also vpossess the ability 'to :pass arelatively-widebandfof-frequenciesfsince'aradio ech'o transmitter 4will contain :side band frequencies extending theoretically "to innity on both sides ofthe 'cari-ier, -thus vrequiring a moderatelywd'e bandi-receiver :order .te obtain va satisfactory replica of the transmitter pulse.

Obviously a lnearby frequency modulated, amplitude -mo dulated :or continuous rwave transmitter vwhose f carrier ffrequency Ifalls .-within the bandwidth of the f-receiver .willdeliver a-comparatively strong signal -to the :receiver antenna which will either block the receptionorfover-ride the .echo. For these .freasons .the Aapplicant has contrived "the present invention.

-It is therefore'an object 'ioffthi's invention .to provide a 'means for "maintaining a ,high :ratio of signal to interference 'in radio reception.

It is another object of this invention to provide a :means in :a lradio receiver :for .balancing out stongiinterferingsignalsandaecepting'weak desired 'si'ghal's.

It is 'another object of thisiinvention "to `provide a means in a radio echo receiverllthatiwillblank the cathode ray tube indicator upon reception of a-'stro'n'g interfering signal. v

other' objects 4of'-trie present;invent-ionwill ibecome 'fappa'r'ent Vvfrom the following Ydescription when taken together with Ithe accompanying drawings, inwhich':

'.Fig. `-1 iis a .schematic diagram Vier'xibodying the principles f lthe present invention;

Fig. 2fisfaschematicidiagram emb'odyinga'medincation fof :the present inve'ntion';

Fig. 3ra) is -Ya graphical representation of 'the voltage Voutputof the tubes vin Figs. 1 "and 2.;

Fig. 3161i) is a graphical representation-of `the resultant Voltagesof Fig. 3(a");

Fig. 4 'is a schematic diagram illustrating the incorporation vof Figs. "1 and `2 lto vform another embodiment -of lthis invention;

Fig. f5 'is anothermodiiication of `Fig. l, and

Fig. f6 is a graphical representation of the voltage'ou'tputs and resultantoutput of the tubes in the circuit-of Fig. l=5.

The interference circuit shown in lscl'iematic detail in Fig. A1 employs two tubes I Vand 2 whose control grids 3 and vIl 'are'connected in push-pull relation 'by the secondary winding of the transformer 5, each having the same grid return resister 6 and `also having 'their fanodes '1 and I8 tied in parallel to the primary windingfo'ftransformer 22. The cathode 9 lrof tube VI is grounded and therefore possesses zero bias, v:while the -catho'de l0 of tube l'2 has, in combination, `ra self-bias through the parallelrcapacitance Il 'and variable resistance `I2 and a f-ii-xed bi-as through the resistance 'I3 and the `B supply. `Capacitance 15 and I6 'are radio .frequency 'by-pass 4eondensers to ground -for 'therscreengrids while 1the 'flxedresistance IFI iis Ia dropping resistor for the kscreen supply ofvtube I. Thevoltagegain'o'ftube 2can be controlledlby cathodeires'istance I2 and screen resistance I4.

The action'of 'the present circuitA may-beunderstood by I considering. a fs'trong interference v'/signal of the proper frequencyy cuttinglthe antenna @I8 at the saine .Jtime that an V:echo -cuts the antenna. Both signalslreceivefequal amplification by vthe radio 'frequency amplier '-I9 Vand Vsubsequently are sent -on to thecOnVer'terfZ' where fthey'are heterodynedtosome lowerffrequency by thelocal oscillator 'S2-I. Cons'ideringgnow, the strong'interfering A signal which Y alternately swings the control grids ofv bothtubes positive andthennegative in phase-opposition. The positivehalf-cyclesfapplied to thegrid 3 fof tube larefclipped bygrid current flowinfg :through resistance J"I5, `but the negative 'half-cycles are amplified vand appear in the tube 4plate -fcircuit '-as positive half-cycles. Conversely, the negative swings of grid 4 of tube2 are clipped-since the gridisalready'strongly negative, 'bult 'fthe positive swings `are amplified and passed on to the output'as1negative pulses. Thus the Youtputs of '.both'tubes I and E2 `are `in phase opposition and will therefore buckfone fanether. The .output from tube I2 l1 can beadjustedeby resistanoes I2 and 'I4 so that it avill'fcancl Vthe loutputirom tube '1I Now:considertheweakfecho signals swinging the control grids in a similar manner. Both the positive and negative swings of grid 4 will be clipped since the negative bias on this grid is so strong that the tube is operating far down on the knee or cut-off portion of the saturation curve. Because of the weak grid swings of tube I there will be a reduction of grid current flowing, consequently dropping tube I down to class A operation thereby amplifying and passing both the positive and negative swings of grid 3. It is therefore apparent that the only signals which successfully reach the output transformer 22 are those of the echo which are passed through tube I. From this transformer the signal is fed into an intermediate amplifier 23, then a detector, a video amplifier and finally the oscilloscope 24.

A more complete understanding of the effective output from this interference reducing circuit may be had by referring to Figs. 3a and 3b. The curve I is a plot of the output versus the input of tube l. The curves 2a, 2b and 2c are plots of the output versus the input of tube 2 under different adjustments of bias. It must be remembered that the curves of tubes lI and 2 in Figs. 3a are 180 out of phase so the resultant output will be that shown by the curves in Fig. 3b. Upon examining this figure it is clearly seen how the combined output from tubes l and 2 may be peaked at different points along the input axis by changing the amount of bias on tube 2. This provides a very effective means of controlling the interfering signals for various signal strengths,

Now to examine the operation of the interference cir-cuit of Fig. 2. This circuit comprises the video amplifier of the receiver, and therefore feeds its output directly to the oscilloscope. The input electrodes of tubes 3| and 32 are connected rto the anode 3l! and cathode 29 respectively of a twin diode 25 which has its other anode and cathode energized by one side of the secondary winding of the intermediate transformer 22. For purposes of explanation consider only the envelopes of both the interfering and echo signals, since this circuit comprises the video stage of the receiver. Considering first, the strong interfering signal, the negative half cycles of which drive the upper section of the twin diode 25 into conduction to thereby impress a rectified negative signal on the grid 33 of tube 3l. This signal is amplified and inverted to appear at the plate of tube 3| as a positive signal. Alternately, the positive halfcycles of the interfering signal drive the lower section of the twin diode into conduction to thereby impress a rectified positive signal on the grid 34 of tube 32. This signal is amplified and inverted to appear at the plate of tube 32 as a negative signal. The h-alf-cycles of the signal occur alternately but the envelopes can be considered as occurring simultaneously. Since anodes 38 and 42 are tied in parallel the output envelopes therefrom will buck one and another, thus by adjusting the variable resistance 40 and potentiometer 4I the outputs can be made to cancel out with the exception of small transients which may occur at the beginning and end of the pulse envelope.

By inserting resistance 28 in the cathode of tube 3l there might be a small amount of degeneration inherent in the tube which would result in the negative output of tube 32 predominating over the positive output of tube 3I. This would simply blank the oscilloscope with the output tied to the intensity grid of the scope.

Now consider the weak echo signal being imparallel.

pressed upon the input of the twin diode 25, the positive half cycles of which drive the lower section of the diode into conduction to thereby impress a positive pulse on the grid 34 of tube 32. This pulse is clipped since the negative bias is so strong and the signal is so Weak. The negative half cycles of the echo signal, however, drive the upper section of the twin diode 25 into conduction to thereby impress a negative pulse on grid 33 of tube 3l. This pulse is amplified and sent on to the intensity grid 26 of the oscilloscope as a positive pulse. At this point it should be noted that the output of the video amplifier can be applied to either the deflecting plate or the intensity grid of the oscilloscope.

Thus by combining the two circuits previously described and shown in Figs. 1 and 2 into a cascade circuit similar to Fig. 4 and comprising the intermediate frequency and video amplifiers of a receiver an excellent system of interference elimination may be realized. This circuit provides excellent control and cancellation of interfering signals up to 5 volts with other signals coming through successfully.

Another scheme of interference elimination is disclosed by the circuit contained in Fig. 5. This circuit contains two tubes 44 and 48 having their grids operating in push-pull and their anodes in A potentiometer 45 is placed across the secondary winding of the input transformer 43 to provide an adjustment of the amount of input each tube receives and thereby governs which tube precedes in the point of saturation. F01` example, consider each tube as having a gain of 20 and the potentiometer 45 being set at a point where tube 44 receives twice the input as tube 48. Tube 44 has a gain of 20 on all signals below saturation while tube 48 has a gain of l0 thus providing a net gain of 10 since the simultaneous outputs of the two tubes oppose each other.

When tube 44 reaches saturation and tube 48 approaches saturation the net gain will decrease accordingly until both tubes are saturated and thereby produce a net gain of zero. The most intense echc signals will not usually drive the tubes to saturation and consequently the resultant output voltage is proportional to the resultant curve of Fig. 6.

A typical set of curves are plotted in Fig. 6 for one setting of the potentiometer. The peaking of the resultant output can of course be varied along the input axis as will be determined by the setting of the potentiometer 45.

Although I have shown and described certain and specific embodiments of the present invention I am fully aware ofthe many modications possible thereof. Therefore, this invention is not to be restricted except insofar as is necessitated by prior art and the spirit of the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

I claim:

l. A means of eliminating interfering signals from electromagnetic wave receivers tuned to receive a signal of predetermined frequency comprising a pair of amplifying tubes each having lan anode, a cathode and a grid, an input transformer having primary and secondary windings, said secondary winding connecting said grids in push-pull relation, lan output transformer also having primary and secondary windings, said anodes connected in parallel and to one terminal of the primary winding of said output transformer, the cathode of one of said tubes tied to ground, a capacitance and variable resistance tying the cathode of the other of said tubes to ground and a means for exciting the primary of said input transformer with said interfering and tuned signals.

2. A means of eliminating interfering signals from electromagnetic wave receivers tuned to receive a signal of predetermined frequency comprising a pair of amplifying tubes each having a cathode, an anode, a control grid and a screen grid, an input transformer having primary and secondary windings, said secondary winding connecting said control grids in push-pull relation, an output transformer also having primary and secondary windings, said anodes connected in parallel and to one terminal of the primary winding of said output transformer, the cathode of one of said tubes tied to ground, a parallel capacitance and variable resistan-ce tying the cathode of the other of said tube to ground, a xed resistance connecting the screen grid of iirst mentioned tube to the anode supply, a variable resistance connecting the screen grid of the other of said tubes to the anode supply and a means for exciting the primary of said input transformer with said interfering and said tuned signals.

3. A means for eliminating interference signals in electromagnetic wave receiver tuned to receive a predetermined desired signal comprising, a pair of amplifying tubes each having at least an anode, a cathode, and a grid, an output means, said anodes being connected in parallel and to said output means, means detecting and impressing the negative half envelopes of both said interfering signals and said desired signal on the grid of one of the said tubes and the positive half envelopes of said interfering signal and said desired signal on the grid of the other of said pair of tubes, and means operative to bias said other of said tubes near cut-off and said one of said tubes near zero.

4. A circuit for suppressing strong unwanted signals from the output of a radio frequency receiver tuned to receive input signals of lesser intensity than said unwanted signals comprising, a pair of amplifying tubes each having grid, cathode and anode electrodes, the anodes of said tubes connected in parallel, a utilization device coupled to said anodes, a phase splitter supplying both the unwanted and the desired signals to the grids of said tubes in push-pull relationship, a bias source connected between the grid and cathode electrodes of one of said tubes to bias said tube to cut-off, whereby only large positive polarity signal voltages are translated by said one of said tubes, and a grid return for the other of said tubes including a grid current limiting resistor connecting the grid of said other tube to its cathode to provide near zero bias therefor Whereby Weak signals of either polarity are amplified by said other tube and large positive polarity signals are grid limited thereby.

5. A circuit for suppressing strong unwanted signals from the output of a radio receiver subject to receive input signals of lesser intensity than the unwanted signals, comprising a pair of amplifying tubes each having at least an anode, a cathode and a grid, means connecting said anodes together in parallel, detecting means connected to the grid electrodes of said tubes, said detecting means being operative to apply the positive detected envelopes of the incoming signals to the grid of one of said tubes and the negative detected envelopes to the grid of the other of said tubes, means biasing said one of said tubes to substantially plate current cut-olf whereby only the large amplitude incoming signals are translated thereby, means establishing 'a near zero bias for said other of said tubes whereby both the large and small amplitude incoming signals are translated thereby, and means for controlling the gain of said one tube to effect substantial amplitude equality in the simultaneous output signals delivered by said tubes.

6. A circuit for suppressing strong unwanted signals from the output of a radio receiver subject to receive input signals of lesser intensity than said unwanted signals comprising, a detector circuit having a pair of output terminals one of which provides detected output signals oi' one polarity and the other of which provides detected output signals of an opposite polarity, a pair of signal transmission channels, one of said transmission channels coupled to one of said detector output terminals and adapted to translate a wide amplitude range of detected output signals, the other of said transmission channels coupled to the other of said detector output terminals and adapted to translate only those detected output signals fed hereto which exceed a predetermined amplitude, and output means coupled to said transmission channels for combining in opposition the output signals derived therefrom.

IRVING H. PAGE.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 1,705,993 Oswald Mar. 19, 1929 1,765,606 Ohl June 24, 1930 1,814,051 McCaa July 14, 1931 1,822,922 Culver Sept. 15, 1931 1,901,929 Peterson Mar. 21, 1933 1,959,275 McCaa May 15, 1934 2,001,622 McCaa May 14, 1935 2,199,820 Gannett May '7, 1940 2,215,946 Radinger Sept. 24, 1940 2,265,269 Dallos Dec. 9, 1941 

